The electrostatic charge that can be taken up by a person is of the order of magnitude of approximately 0.6 μC. The person can be simulated by a capacitor having the capacitance of 150 pF. If the charge of 0.6 μC is stored on a capacitor having the capacitance of 150 pF, then this corresponds to a charging voltage of approximately 4 kV. If a person who has been charged to such a voltage touches a grounded object, an electrostatic discharge occurs. The latter proceeds in approximately 0.1 μs with currents of up to several amperes.
Due to the small oxide thickness and the small dimensions of the interconnects and pn junctions, electrostatic discharge processes proceeding via MOS (Metal Oxide Semiconductor) components can lead to the destruction of the device. The discharge processes primarily lead to the breakdown of the gate oxide or else to the overheating of pn junctions or interconnects. The energy converted during an electrostatic discharge is of the order of magnitude of 0.1 mJ and is therefore not very high. However, if this energy is fed in pulsed fashion into a volume of the order of magnitude of a few cubic micrometers, then this can give rise locally to such a high temperature that the silicon melts. Output terminals are generally less sensitive than input terminals since the output driver transistors have a large energy absorption capacity. Input terminals are connected to the gate terminals of the input transistors. The thin layers of the gate oxide can easily be destroyed in the case of electrostatic discharge. Input terminals of an integrated circuit should therefore have ESD (electrostatic discharge) protection circuits. The ESD protection circuits must have high resistance for input voltages that lie within the specification. They should have low resistance for voltages that lie outside the specification and, in particular, in the ESD range.
In a known circuit arrangement for protecting integrated circuits against electrostatic discharge, protection diodes, so-called ESD diodes, are used. The cathode terminal of the diode is connected to an input terminal of the integrated circuit at which the occurrence of a high electrostatic voltage is to be expected. The anode terminal is connected to a terminal for the reference potential. If positive voltages that lie outside the specification occur at the reference potential terminal, then the diode is forward-biased and dissipates the positive electrostatic charge to the input terminal of the integrated circuit that is connected to its cathode.
If a high positive electrostatic voltage occurs at one of the input terminals of the integrated circuit, then the ESD diode is operated in the blocking range. At sufficiently high voltages, lying between 7 V and 12 V for example, a breakdown occurs at the diode on account of the avalanche effect. The diode has become conductive. The electrostatic charge present at the input terminal of the integrated circuit is dissipated via the diode path to the terminal for the reference potential of the integrated circuit.
In particular during the production of semiconductor memories, diodes are not fabricated in a production process provided especially for them. They arise as by-products during the production of transistors within integrated circuits. The drain-substrate diodes that form between drain and substrate shall be mentioned as an example in the case of transistors. The behavior of such diode structures is not controlled during the production process. The breakdown behavior of the diode structures is therefore known only to an approximation. What is problematic is that the voltages required for the diode breakdown are often so high that the components of the integrated circuit are already destroyed before the diode breaks down in the reverse direction. This applies primarily when using MOS circuits having thin gate oxides.